This invention relates to antimicrobial peptides and, more specifically, to cryptdin peptides and their uses.
Survival in a world teaming with microorganisms depends on a network of host defense mechanisms. Among these mechanisms are phagocytosis in which cells which circulate in the blood system, ingest and digest potentially harmful microbes. Although pathogenic microbes may vary considerably, phagocytes are able to destroy the vast majority by sequestering them in intracytoplasmic vacuoles and exposing them to a lethal mixture of organic and inorganic toxins.
Perhaps the most remarkable ultrastructural feature of phagocytes are their several thousand cytoplasmic granules, which are membrane-bound organelles typically about 0.3 .mu.m in diameter. During phagocytosis, some of these granules fuse to phagocytic vesicles thus enabling the contents of the granule to enter the lumen of the vesicle. Early observers surmised correctly that the granules contained factors which were responsible for intraphagosomal killing in digestion of microbes. These granules contain a mixture of antimicrobial molecules including various peptides such as the so-called defensins.
Defensins are abundant antimicrobial peptide components of vertebrate neutrophil and macrophage granules. Members of the defensin family have been identified previously in human, rabbit, guinea pig, and rat phagocytes, primarily those phagocytes termed phagocytic granulocytes. Defensins are cationic peptides, generally between 3 and 4 kD in size which exhibit broad-range antimicrobial activities against gram negative and gram positive bacteria, many fungi, and some enveloped viruses. The peptides are characterized by eight invariant amino acids including six invariant cysteine residues which constitute a unique disulfide motif. The three disulfides stabilize a tertiary conformation consisting predominantly of beta-sheet. The highly ordered structure and the absence of helix make defensins unique among known antimicrobial peptides. It appears that defensins exert their antibacterial effect by permeabilizing the cytoplasmic membrane of the target microorganism by a mechanism that may involve the formation of ion channels.
Until recently, defensins had been identified only from circulating or tissue phagocytes of myeloid origin. However, it has been surmised that similar peptides might be present in the epithelial cells of the small intestine, based on the presence of a particular mRNA. Because of the importance of the small intestine in preventing access to the systemic circulation, peptides whose activity would be effective in the small intestine, either within the cells of the epithelium or in the intestinal lumen, could provide an important therapeutic or prophylactic mechanism. The present invention provides such peptides, allowing such treatment, and providing additional benefits as well.